This paper reviews key scientific issues related to the performance assessment of engineered barrier systems. In contrast to the inherent uncertainties associated with inhomogeneous geologic barriers, engineered materials can be tested over a wide range of conditions and thus reliable predictions can be made for a range of scenarios. The two factors which dominate near-field performance are the pore-water composition and mass transfer constraints. A number of examples are presented which demonstrate that mass transfer analysis provides a sound scientific basis for near-field performance assessment. In particular, detailed Waste Package models, which integrate the performance of multiple barriers, are important for optimizing engineered barrier designs and for guiding research.